Phase Change Material (PCM) is suitable for storing thermal energy in the form of latent heat. Microwave energy is changed to heat as soon as it is absorbed by a microwave susceptor.
Every microwave oven contains a magnetron, a tube in which electrons are affected by magnetic and electric field in such a way as to produce micro wavelength radiation at about 2.45 GHz. In commercial microwave oven, the oven has a power input of about 1000 watts of alternating current. As these microwaves generated from magnetron bombard the PCM, they cause the polar molecules to rotate at the same frequency millions of times a second. This agitation creates molecular friction, which heats up the PCM.
U.S. Pat. No. 5,070,223 discloses an article of apparel in which microwave sensitive material is present in only one portion. The overheating generated at this portion heats the whole article.
U.S. patent application Ser. No. 11/511,775 describes the use of carbon black as conductive particulate material in nano scale. Our invention does not require carbon black in such a minute particle size.
U.S. Pat. No. 5,310,977 demonstrates a metalized film susceptor with aluminum metal layer patterned to form a film which produces too much heat locally.
U.S. Pat. No. 5,630,961 discloses a microwavable thermal storage composition which is free flowing powder. However this patent does not tell about cycling stability of Phase Change Material (PCM). It also does not tell whether it remains in free flowing powder form after one melting and freezing cycle or not.
U.S. Pat. No. 4,962,000 discloses a composite with two microwave absorbing zones which are being separated by a microwave transparent layer with a pressure sensitive adhesive. This also discloses that composite is incapable of arcing but doesn't mention anything about overheating.
Further, other prior art approaches are also subject to many variations. For example, U.S. patent application Ser. No. 11/906,730 discloses a phase transition golf ball in which a portion of golf ball comprising microwave susceptor is heated more rapidly to a higher temperature in comparison with a portion of golf ball that does not contain microwave susceptor. This shows that susceptor is accumulated at one point to heat the whole article.
In normal microwavable Phase Change Material (PCM), people face many problems after using it for a number of times. After some melting cycles the microwave susceptor becomes inactive. Thus the Phase Change Material (PCM) takes more time to charge or does not charge at all. Heat generated by microwave gets concentrated at a few points and then this concentrated heat is used to melt the whole Phase Change Material (PCM). Due to this degradation of Phase Change Material (PCM) occurs and essentially from these points heat is transmitted by convection only, thus leading to spots of very high temperature or non-uniformity of temperature all over the surface. Also due to use of high percentage of microwave susceptor, latent heat value of microwavable Phase Change Material (PCM) decreases.
Microwavable hot or cold packs which are commercially available are water based and in liquid phase. Phase transition does not occur after microwave heating. It remains in only liquid state. Thus only sensible heat is used for heating application which is 2-3 times less than Phase Change Material (PCM).
Therefore there exists a need for a microwave sensitive Phase Change Material (PCM) which is more effective and efficient.
The novelty in the present invention resides in providing a new form of microwave susceptor incorporating a Phase Change Material (PCM) and a binding agent like clay. This new composition prevents the agglomeration of the susceptor. Also there will be uniform heating (charging) and discharging—again because of no agglomeration or sedimentation of susceptor, thus no Phase Change Material (PCM) degradation. Uniform distribution of susceptor means lower amount can be used. Thus latent heat of resultant Phase Change Material (PCM) composition is high according to the present invention. No change in workability/efficiency of Phase Change Material (PCM) occurs, even after several charging and discharging cycles. Phase Change Material (PCM) takes only 2-3 minutes time depending on quantity for melting/charging in 1000 watt domestic microwave. For example if we take 100 grams of microwavable Phase Change Material (PCM), on exposing to microwave irradiation in domestic microwave oven ing, it melts in just 30 sec to 60 sec time duration.